It is known in the prior art to upgrade hydrogen deficient petroleum oils to more valuable products by thermal and catalytic cracking operations in admixture with a hydrogen donor diluent material. The hydrogen donor diluent is a material, aromatic-naphthenic in nature that has the ability to take up hydrogen in a hydrogenation zone and to readily release hydrogen to a hydrogen deficient oil in a thermal or catalytic cracking operation.
One advantage of a hydrogen donor diluent operation is that is can be relied upon to convert heavy oils or hydrogen deficient oils at relatively high conversions in the presence of catalytic agents with reduced coke formation. Coke as formed during the cracking operation is usually a hydrocarbonaceous material sometimes referred to as a polymer of highly condensed, hydrogen poor hydrocarbons.
A great demand continues for refinery products, particularly gasoline, fuel oils, and gaseous fuels. Because of the shortage of high quality, clean petroleum-type feedstocks, the refiner now must turn to heavier, more hydrogen-deficient, high impurity-containing cracking feedstocks. Included in this category are heavy vacuum gas oils, atmospheric residua, vacuum tower bottoms, and even syncrudes derived from coal, oil shale, and tar sands, and even coal itself.
In some cases, high levels of nitrogen and sulfur constitute a serious problem in such refractory, low-crackability stocks, particularly with reference to down-stream processing and product environmental and pollution limitations. An even more difficult problem is posed by the presence of metallic impurities, nickel, vanadium, iron, etc., preserved through geologic time in heavy petroleum fractions. Such metals, commonly associated with porphyrin rings and asphaltenes in high molecular weight cuts, can cause serious engineering/hardware problems in catalytic cracking. As catalyst is exposed to repeated cycles of reaction/regeneration in a fluid cat cracker (FCC), these metals are absorbed and tend to build up with time and accumulate on the catalyst. They then cause dehydrogenation-type reactions, resulting in formation of very large amounts of coke, large amounts of H.sub.2 gas, which may put a severe strain on the FCC unit regenerator air blower and wet gas compressor capacity. Further, and very important, their presence is often associated with a serious loss of conversion and gasoline yield.